Selkies are a type of faery. They are beautiful women who wear seal skins to swim in the ocean. If a man can steal the skin, he can marry the selkie. But if the selkie ever finds the skin, she will always go back to the ocean, deserting him no matter how much she may love him, even deserting her children, because the love of the sea is stronger.
The song "Selkie" on Unrepentant Geraldines is about a selkie who willingly gives up her seal skin for the man she loves. Because her love for him is stronger than even her love of the sea.
Someday I hope to find someone I love that much. Just a bit of trivia for you.